1. Field of the Invention
The present invention relates to storage and production processes for entrapping immobilization pellets. More particularly, the present invention relates to storage and production processes for entrapping immobilization pellets suitable for wastewater treatment in order to remove nitrogen.
2. Description of the Related Art
Conventionally, as a process for treating wastewater such as sewage, industrial wastewater or agricultural wastewater, a biological process has been widely used, because the process involves a low cost as compared with a physicochemical process. Typical examples include an activated sludge process used for sewage. However, since slowly growing microorganisms such as nitrifying bacteria in activated sludge which are highly involved in removing organic substances or nitrogen in wastewater flow out of a reaction tank without sufficient growth, the reaction speed is significantly decreased particularly when the water temperature is low, resulting in deterioration of water quality. In this situation, a wastewater treatment process using attachment immobilization pellets in which microorganisms are attached to and immobilized on resin systems and plastic pellets (see Japanese Patent Application Laid-Open Nos. 2002-159985 and 2004-41981, for example) and a tap water treatment process using biologically activated carbon (see Japanese Patent Application Laid-Open Nos. 2003-200183 and 10-290992, for example) have been practically used for retaining microorganisms such as nitrifying bacteria in a treatment tank at a high concentration and stably. There has also been performed a wastewater treatment process comprising producing entrapping immobilization pellets in which microorganisms such as nitrifying bacteria are entrapped and immobilized in an immobilizing material, and packing a wastewater treatment tank with the entrapping immobilization pellets to increase the concentration of nitrifying bacteria.
However, in the wastewater treatment process using attachment immobilization pellets, the attached microorganisms are detached from the pellets, or microorganisms differing from target microorganisms such as nitrifying bacteria are attached to the pellets, making it difficult to sufficiently retain the target microorganisms. In contrast, in the wastewater treatment process using entrapping immobilization pellets, target microorganisms can be acclimatized in the pellets and thus can be retained in the pellets at a high concentration. Accordingly, wastewater can be treated fast.
Such entrapping immobilization pellets are produced by tube forming, dropping granulation, sheet forming or the like. Tube forming is a process comprising injecting a mixture of microorganisms with a polymer material into a vinyl tube having a diameter of several millimeters to polymerize and extrude the mixture, and cutting the mixture into cylindrical pellets with a certain length. This process can provide pellets with high form accuracy, but is not suitable for mass production. Dropping granulation is a process comprising dropping a mixture of microorganisms with a polymer material into a separate liquid to produce spherical pellets. This process allows easy mass production, but has a drawback in that the produced pellets have various particle sizes. Sheet forming is a process comprising forming a mixture of microorganisms with a polymer material into a sheet and finely cutting the sheet into rectangular pellets (see Japanese Patent Application Laid-Open No. 2003-235553). Entrapping immobilization pellets are often produced by sheet forming for wastewater treatment equipment requiring a large amount of pellets, because this process provides high form accuracy and is suitable for mass production.
However, since oxygen in air generally reacts with radicals, contact of an immobilizing material with air causes suppression of immobilization of microorganisms on the immobilizing material. Sheet forming provides a large contact area between an immobilizing material and air, and thus has drawbacks in that pellet strength is decreased due to unstable polymerization, and COD flows out due to the remaining unpolymerized material when entrapping immobilization pellets are introduced, for example.
As a device to solve these drawbacks, the present inventors have filed a patent application for block forming. The block forming is a process comprising injecting and immobilizing a liquid mixture of microorganisms with an immobilizing material (polymer material) in an almost cubic or rectangular solid forming frame to reduce the contact area between the immobilizing material and air. This can suppress a decrease or variation in pellet strength or the presence of the remaining unpolymerized material due to contact of the immobilizing material with air.
Typically, such entrapping immobilization pellets are granulated by forming a pellet into a sheet or block and cutting the pellet into about 3 mm squares in a production plant (granulation plant), and then stored and transported to a treatment tank into which the entrapping immobilization pellets are to be introduced.
However, in the conventional process for storing and transporting entrapping immobilization pellets, entrapping immobilization pellets brought into contact with each other are easily caused to adhere to each other and aggregated into balls, resulting in a decrease in wastewater treatment efficiency. For this reason, when transported, entrapping immobilization pellets must be dipped in water whose weight is almost equal to that of the entrapping immobilization pellets. This involves a heavy work load, disadvantageously, because about twice the weight of necessary entrapping immobilization pellets must be transported.
When storing and transporting a pellet block still to be cut into pellets in normal air for a long time as is in order to reduce the above-described work load during storage and transportation, only the outside of the pellet block is dried, and the aqueous content differs between the inside and the outside of the pellet block, making the pellet block deformed, disadvantageously. Moreover, microorganisms have increased activity and thus methane, hydrogen sulfide or the like is generated in the pellet block to produce cracks in the pellet block, resulting in a variation in shape or quality of the entrapping immobilization pellets obtained by cutting the pellet block, disadvantageously.
The present invention has been achieved in view of such circumstances. An object of the present invention is to provide storage and production processes for entrapping immobilization pellets which can stably store and transport a pellet block and can provide entrapping immobilization pellets having high quality stability.